The present invention relates to a method for making phenolic chain-stopped polycarbonate which is substantially free of diarylcarbonate. More particularly, the present invention relates to the employment of a phase transfer catalyst during the phosgenation of bisphenol to produce chloroformate terminated polycarbonate, followed by the addition of an amount of phenolic chain-stopper substantially equivalent to the chloroformate terminal groups.
Experience has shown that during the preparation of polycarbonates by the phosgenation of bisphenols under interfacial reaction conditions utilizing aromatic hydroxy chain-stoppers, such as phenol, diarylcarbonate is often formed as a by-product. Because diarylcarbonate can plate out during injection molding procedures, it represents an undesirable contaminant in certain polycarbonate applications, such as CD grade polycarbonate.
As shown by U.S. Pat. Nos. 4,737,573, 4,743,676 and 5,043,203, various techniques have been used to minimize the formation of diaryl carbonates during phosgenating procedures for making end-capped polycarbonates. In one procedure, a bischloroformate oligomer is prepared utilizing an excess of 1.1-3.0 moles of phosgene, per mole of bisphenol. A phenolic chain-stopper, such as phenol, and a condensation catalyst can be added to the mixture. In another procedure, a phenolic chain-stopper is added to a bischloroformate oligomer to produce a partially capped bischloroformate, followed by the addition of an interfacial polycarbonate condensation catalyst. A further procedure for making aromatic polycarbonate substantially free of diarylcarbonate is by forming an oligomeric bisphenol monochloroformate, and then incorporating an end-capping phenol, tertiary organic amine and additional alkali metal hydroxide into the mixture.
Although several prior art procedures have been found effective for minimizing diarylcarbonate formation, experience has shown that an excessive amount of phosgene or chloroformate end groups are necessarily consumed before polycarbonate molecular weight build-up can be achieved. It would be desirable therefore to provide a method for making polycarbonates substantially free of diarylcarbonate without excessive consumption of phosgene, chloroformate end groups or both.